The increasing output of laser oscillators has led to an expansion of the fields of application for laser beam welding, which uses laser beams in place of the conventional gas welding or electrical welding. In particular, there is a strong demand for the capability to weld superimposed steel sheets and the like in the automobile industry. Namely, the steel sheets used in the automobile industry are zinc-plated steel sheets treated with zinc plating for the purpose of rust prevention, and a stable and high-speed laser beam welding of such zinc-plated steel sheets and the like is now required.
When welding superimposed zinc-plated steel sheets, however, a surface plating layer on the steel sheet is evaporated when irradiated with a laser beam, causing the generation of many blowholes. In particular, if there is no gap between the steel sheets, the steam of evaporated zinc blows off the molten base metal, making welding impossible. This is considered to be because zinc has a lower boiling point than the melting point of iron, and because the evaporating pressure is high, thus causing the molten iron to be blown off.
Accordingly, when welding superimposed zinc-plated steel sheets, a gap of few hundred microns is provided between the steel sheets to allow the zinc steam generated on the surfaces thereof to escape. Nevertheless, it is extremely difficult to maintain the gap at a fixed value, at an actual production site, and thus the laser beam welding must be carried out while the steel sheets are in close contact with one another.